With the reduction of feature size in complementary metal oxide semiconductor (CMOS) processes, supply voltage requirements decrease. This higher integration density provides that each digital and analog cell in a mixed signal very-large-scale-integration (VLSI) circuit consumes less power. There is a benefit to the digital cells in that performance improves; however, analog cells suffer from the reduction in size since minimum size transistors cannot be used due to noise and offset requirements. Low supply voltage requirements dictate a need for complex circuit solutions, which in some cases result in performance degradation. Thereby efficient topologies that combine low-voltage operation with high power efficiency and small die area is the solution for analog cells.
An improved class AB control circuit is presented in the following publication: “Compact Low-Voltage Power-Efficient Operational Amplifier Cells for VLSI” by Klaas-Jan de Langen and Hohan H. Huijsing-IEEE Journal of Solid-State Circuits, Vol. 33, No. 10, October 1998 pages 1482-1496 which is incorporated by reference herein. A suitable class AB control circuit having a low number of components and low current consumption is shown in FIG. 1 and is similar to the control circuit shown in FIG. 7 of the foregoing reference. The op-amp has a PMOS input stage MPINM1 and MPINP1, allowing common-mode input voltage down to the negative rail. The input stage drives a cascoding and summing circuit including a current mirror MN8 and MN9, and cascodes MN10, MN11, MN12, MP5, MP6, and MP7. The summing circuit drives the output stage MP8 and MN15 and the class-AB control circuit MP9, MP10, MP11, and MN14. The class AB control uses the simple minimum selector MP9, MP10, and MP11. In operation, the current of transistor MN15 is measured by transistor MN14. The drain current of transistor MN14 flows through mirror MP9 and MP10, which is a part of the minimum selector circuit MP9, MP10, and MP11. Transistor MP10 of the minimum selector circuit operates mainly in the linear region. Only when transistor MP8 handles large output current does transistor MP10 operate in saturation. Transistor MP11 measures the current of transistor MP8 and is also part of the minimum selector. The drain current of transistor MP11, which is the output of the minimum selector, flows through transistor MN7 and steers the class-AB amplifier cascode MN10, MN11, MN12, MP5, MP6, and MP7.
This architecture is capable of having a supply voltage VCC as low as the sum of twice the drain voltage at saturation (2Vdsat) and the gate-to-source voltage Vgs, wherein VCC=Vgs+2Vdsat. This architecture is one of the several known architectures that have the capability of having a lower supply voltage and having a double cascode high impedance node. Moreover, the disclosed class AB control circuit does not introduce additional non-dominant poles as with other known implementations. As a result, this architecture has the best AC performance. Accordingly, the required power for a given bandwidth and phase margin is minimized.
The class-AB control circuit within the amplifier shown in FIG. 1, however, has a significant disadvantage. Specifically, when the output of the amplifier clips to ground or the negative rail, a surge of current is pulled from the power supply. Accordingly, NMOS device, transistor MN15 goes into the linear triode region because its drain-to-source voltage Vds is lower than what is necessary for it to stay in saturation. Accordingly, sensing NMOS device, MN14, remains in saturation. Accordingly, when transistor MN15 goes into the linear region for a given amount of drain current, the gate-to-source voltage Vgs of transistor MN15 is much higher in the linear region. Thus, since transistor MN14 is still in the saturation region, it has been conditioned to expect that transistor MN15 will send substantially less current than when it is does during output clipping. Thereby, the gate-to-source voltage Vgs of transistor MN15 causes a significantly larger amount of current to go through transistor MN14. Accordingly, the ratio of the currents supplied to transistors, MN14 and MN15, prior to the output of the amplifier being pulled low is 10/1. Yet, as a result of the output clipping, this same ratio changes to approximately 10/0.01. The drain of transistor MN14 connects to the diode connected transistor MP9. Transistor MP9 provides as much current as it can to transistor MN14. The only limitation of current supplied by transistor MP9 is when the gate-to-source voltage Vgs of transistor MP9 is too large, wherein there is little voltage remaining if the gate-to-source voltage Vgs is subtracted from the supply voltage. There, however, must be enough gate-to-source voltage Vgs to keep transistor MN14 ‘on’. Thereby, there exists a supply dependency, wherein when the supply voltage is low, there will be less current surge but it will be on the order of 4 microns to 1.8-2.7 milliamps. This poses a significant problem. Furthermore, when the output voltage is below the drain voltage Vdsat for transistor MN15 at saturation, the gain of the summing circuit is dramatically reduced causing the voltage at the gate of transistor MN15 or node N17 to increase significantly. As a result, transistor MN14 is biased at a DC point where a significant amount of additional current flows through transistor MN14 on the order of several milliamps. The exact amount of additional current depends upon the power supply wherein a higher supply will generate a higher current.
A known approach to minimization of surge current produced by the class-AB control circuit within the amplifier has been to add a resistor between the drain of transistor MN14 and the drain of transistor MP9 to cause a voltage drop which will lower the amount of surge current. This approach, however, is not an adequate solution, in that it minimally limits the surge current; yet, a substantial amount of surge current exists. Additionally, while this approach reduces surge current, the value of the resistor varies with process, voltage, and temperature. Moreover, the minimized surge current remains power supply dependent.
Thus, there exists a need for a low voltage amplifier having a class-AB control circuit which generates minimal or no surge current when the output of the amplifier is driven low, in the case where the amplifier is used as a split supply.
The present invention is directed to overcoming, or at least reducing the effects of one or more of the problems set forth above.